It is known to provide a governor system for the fuel pump of an internal combustion engine which is used to power a vehicle, such as an agricultural vehicle or a road vehicle. For vehicles which are used to move heavy loads, it is the usual practice to provide a so-called all speed governor system, since the fueling characteristic provided by such a system is ideal for use when the vehicle is in a loaded state. In such a system, the driver of the vehicle sets the required engine speed (and hence the ground speed) with the vehicle throttle, and the all speed governor system adjusts the fuel supply to the engine so as to attain and maintain the required speed (within the power capability of the engine and any other restraints placed upon the system).
A typical prior art all speed governor fueling curve family is illustrated in FIG. 1. The graph shows the power developed by an engine versus engine speed for any particular throttle position maintained by the driver. The fueling to the engine to produce these power curves is substantially linearly proportional to the power produced. The curve 10 represents one hundred percent (100%) (or maximum) fueling of the engine. An existing problem with prior art all speed governors, such as in engines used in agricultural applications, is the so-called lug back characteristics of the governor at partial throttle operation. For example, the engine may be operated at seventy percent (70%) throttle at engine speed A, which will place the all speed governor at position 12 on the diagram of FIG. 1. If the vehicle is pulling an agricultural implement, such as a plow, and the implement encounters increased resistance, such as a patch of hard earth, this will have a tendency to slow the vehicle and hence slow the engine speed to the point B. This will place the all speed governor at position 14 on the 70% throttle curve. It will be noted that the engine is now producing more power in order to return the engine speed back to point A. Further slowing of the vehicle (i.e. as a result of increased resistance within the earth) will cause the engine speed to slow to point C, placing operation of the all speed governor at point 16 on the 70% throttle curve. At this point, the 70% throttle curve is limited to the 100% fueling curve 10. Any further resistance on the agricultural implement will cause the vehicle to slow further, which will actually result in the engine producing less power, thereby slowing further, which causes the vehicle to produce less power, et cetera. The perception of the driver of the vehicle is a rapid slow down of the vehicle and a perceived loss of power. The driver will have no other choice than to downshift or to lift the implement out of the ground.
The problem with this all speed governor characteristic occurs in the region 18. Because of the sharp transition at the point where the partial throttle curve intersects the 100% fueling curve, the driver receives no warning before the vehicle begins to slow down rapidly and lose power. As the governor transitions from engine speeds A to B to C, the engine continues to increase its power output in an attempt to regain the original engine speed. However, any further reduction of engine speed causes the engine to lose power, therefore causing the vehicle to slow down rapidly.
The main concern with this form of operation is that no warning is given to the driver before the all speed governor transitions to the 100% fueling curve 10 and the vehicle begins to abruptly lose power. Once on the 100% fueling curve 10, further depression of the throttle will have no effect in increasing the speed of the vehicle. Therefore, the undesirable result of the all speed governor is that it causes the engine to increase power as the vehicle slows until it reaches the intersection 16 with the 100% fueling curve, at which point it suddenly begins to lose power, without any advance warning to the driver. There is therefore a need for an engine governor which does not produce this undesirable result. The present invention is directed to meeting this need.